There is every reason to believe that the Mets are going to make a serious run at retaining Jose Reyes this offseason, but as free agency has proven time and again, you can never tell what’s going to happen once a player hits the open market.

Their shortstop of the future, Wilmer Flores, is a promising hitter whose defense is lacking, and he may well break in elsewhere, like second base, when he does come up.

The Mets are also making noises about shifting Ruben Tejada to short if Reyes leaves, which says something about the strength of this free-agent group.

The bad news is that there are precious few shortstops entering free agency in their prime. Excluding Reyes, the average age is an eye-opening 35 years old, which is when shortstops in particular are well on the downside of their careers, especially defensively, as they lose the range and speed that is critical to their position.

So, as an exercise in preparing for the worst, what follows is a list of the best alternative options at short should Reyes leave.

Atlanta Braves’ Alex Gonzalez

The 34-year-old Gonzalez had a lousy year in Atlanta, is terrible at getting on base and strikes out a ton. Sounds enticing, right? He plays good defense, and could probably be had for around for a relatively cheap $2M.

This is a good example of an aging player whose best years, which were not that good to begin with, are almost certainly behind him. The Braves don’t have a major league-ready prospect at shortstop, so they may try to sign Gonzalez on the cheap for 2012.

Milwaukee Brewers’ Yuniesky Betancourt

The Brewers are holding a $6 million option on Betancourt that they will most likely not exercise. He’s an average hitter (.252, 13 homers, 68 RBI in 2011), but is a butcher in the field, having committed 21 errors en route to a terrible .965 fielding percentage.

At 29, Betancourt is one of the youngest options, but signing such a marginal player after losing one the stature of Reyes would be a hard sell to Mets fans.

Philadelphia Phillies’ Jimmy Rollins

For Mets fans, this is certainly the most disagreeable option to ponder, given Rollins’ provocative comments towards the team and its fans over the years. Rollins is a solid fielder, but his speed and his offensive numbers took a sharp drop this year.

He is also looking for at least a three-year deal, just as he is entering his mid-30s, and will likely wait until Reyes establishes the market, as Rollins is generally considered the second-best shortstop available. Look for him to stay in Philly.

St. Louis Cardinals’ Rafael Furcal

Next to Reyes and Rollins, the 33-year-old Furcal is the most talented offensive shortstop out there. The Cardinals have a $12 million option on the shortstop that they will almost certainly not exercise.

Furcal has said that he loves playing in St. Louis, and may take a big discount to stay there.

On the negative side, he has been injury-prone over the last few years, and his speed and defense are both in decline. But look for the Mets to show interest if the Cards don’t make an offer.

Boston Red Sox’ Marco Scutaro

The most intriguing option of the lot, Scutaro is exactly the kind of high on-base-percentage player that GM Sandy Alderson likes.

Boston is a little log-jammed at shortstop, with Scutaro, Jed Lowrie and top prospect Jose Iglesias, who is not quite ready for the big time. But Scutaro and the Red Sox are both holding options for 2012, and it seems likely that one will be exercised.

Detroit Tigers’ Ramon Santiago

The 32-year-old Santiago is a slick fielder who had a solid year, and hit .375 in the ALCS. He splits time between shortstop and second base, and played more at second this year due to the Tigers’ acquisition of shortstop Jhonny Peralta.

He has said he wants to return to Detroit, but also wants to play regularly. I could see the Mets making a move on Santiago if they can get him at a reasonable price.